The proposed neuropsychological model suggests a new and important hypothesis about the attention deficit in Attention Deficit Disorder (ADD) and strongly implicates dysfunction in the right hemisphere (RH). This model proposes that the deficit results in momentary lapses of sustained attention (i.e., over seconds) rather than a long-term deficit (i.e., over minutes). The proposed studies will investigate the development of lateralized maintenance of attention in children and adults with and without ADD. Experiment I will obtain normative data to show that the RH serves an endogenous attentional alerting function. Three age groups (10, 14, 18-20 year-olds) of NonADD (NADD) individuals will be given a lateralized test of sustained attention. Experiment 2 will provide a direct test of the model by allowing a comparison of ADD and NADD children and adults on a lateralized attention task. We hypothesize that individuals with ADD, unlike NADD individuals, will not show RH superiority for lateralized maintenance of attention and providing exogenous cues to reorient attention will produce equivalent performance for ADD and NADD groups. This work will make several important contributions to the field. First, it will contribute to the growing number of studies attempting to link the behavioral symptoms of ADD to brain functioning, thereby providing a new framework for interpreting and studying attentional problems in ADD. Second, the results could have important implications for understanding the developmental course of ADD and the persistence of attentional problems into adulthood. Third, the results may have implications for treatment strategies for ADD. Finally, it will provide important information regarding the development of normal and dysfunctional attention.